Work proposed is to continue to use the digitonin reaction as a basis for localization of free cholesterol with the electron microscope. A major effort will be to define the specificity of the reaction in tissue such as liver, gallbladder and adrenal. Earlier work has emphasized use of digitonin for sterol localization in model systems such as myelin and gallstones. Our most recent studies indicate the need for use of chromatographically purified He-digitonin for autoradiographic techniques. The purified saponin will be used in subsequent localization experiments involving the tissues noted above, as well as in a system of microorganisms grown with and without cholesterol. In addition, gallbladders and livers of mice fed a gallstone-inducing diet for long periods (6 months to one year) will be studied and compared to those of normal animals. Studies are also planned for analysis of degeneration and regeneration of myelin in peripheral nerves of rats given cholesterol-inhibiting drugs while fed a modified cholesterol-free diet. BIBLIOGRAPHIC REFERENCE: Saland, L.C. and L.M. Napolitano 1975. Transmission electron microscopy of diet-induced mouse gallstones. Anat. Rec. 181: 470 (Abstract).